1. Field of the Invention
This invention relates to a process for producing alkadienes by dimerization of butadiene or isoprene.
2. Description of the Prior Art
It is known that contacting an acyclic conjugated diolefin such as butadiene with a catalyst such as platinum, a platinum compound, palladium, a palladium compound or the like in the presence of a reducing agent in either a polar solvent or a nonpolar solvent yields the corresponding 1,6- and 1,7-octadiene (U.S. Pat. Nos. 3,732,328 and 3,823,199). U.S. Pat. No. 3,732,328 states that such a trisubstituted phosphine as triphenyl phosphine, as well as copper salts, are useful for enhancing the catalyst activity and for prolonging the catalyst life if the dimerization reaction is carried out in a steel reaction vessel. It is also known that the addition of an excess amount of the phosphine in the reaction system for telomerizing butadiene leads to a decrease in reaction rate (J. Organometallic Chemistry 49 473 (1973) and ibid. 137 309 (1977)). For this reason, in the examples of U.S. Pat. No. 3,732,328, the phosphine is used in an amount of at most 2 moles per gram atom of platinum or palladium.
When phosphine is not used in excess, however, the platinum or palladium metal complex in solution is extremely unstable against heat, which leads to a decrease in catalytic activity due to thermal decomposition of the metal complex and deposition of metallic platinum or palladium on the wall of the distillation vessel when the reaction mixture is subjected directly to distillation. Accordingly, for the purpose of stabilizing the catalyst against heat in the known processes, it is necessary to use the phosphine in excess amounts at the sacrifice of catalytic activity. Further, in cases wherein distillation is employed to separate the desired reaction product from the reaction mixture containing the catalyst, high-boiling by-products accumulate in the reaction system as the catalyst is reused, and as a result the catalytic activity is decreased.
For the commercial production of alkadienes by the dimerization of acyclic conjugated diolefins, it is particularly important to maintain the catalytic activity and to separate the catalyst from the reaction mixture in such manner that the catalyst does not decompose nor decrease its activity and can easily be reused in the dimerization reaction.
It has now been found that the above problems can be solved efficiently in accordance with the present invention in a very simple manner by (1) contacting butadiene or isoprene with at least one catalyst selected from the group consisting of platinum and palladium and compounds of said metals in a sulfolane solution, in the presence of a tertiary lower alkylamine formate and at least one phosphine compound of the general formula (I) ##STR3## wherein R.sup.1 is a substituted or unsubstituted hydrocarbon group of 1 to 10 carbon atoms; R.sup.2 is a hydrogen atom, a saturated aliphatic hydrocarbon group of 1 to 5 carbon atoms, a nitro group or a halogen atom; m is equal to 1, 2 or 3; n is equal to 0 or 1; x is equal to 0, 1 or 2; y and z each is 0, 1, 2 or 3 (provided that y and z are not concurrently equal to 0 and that x+y+z=3); A is --SO.sub.3 M wherein M is a cation selected from the group consisting of H, alkali metals, alkaline earth metals and NH.sub.4 or the formate or an inorganic acid salt of ##STR4## wherein R.sup.3 and R.sup.4 each is a saturated aliphatic hydrocarbon group of 1 to 4 carbon atoms, in an amount of 4 to 200 moles per gram atom of the metal constituting the catalyst to form dimeric alkadienes; (2) separating the reaction mixture into an alkadiene-containing layer and a catalyst-containing layer; and (3) recycling said catalyst-containing layer to the alkadiene production process.
The essential features of this invention reside in conducting the dimerization reaction in a sulfolane solution and in the use of both a tertiary lower alkylamine formate and at least one phosphine compound of the general formula (I) in an amount of 4 to 200 moles per gram atom of the metal as the catalyst. By virtue of these features, the catalytic activity can be maintained over a prolonged period of time in spite of using the phosphine compound in excess. The catalyst and the reaction product can be easily separated from the reaction mixture and the catalyst so separated, exhibits almost the same catalytic activity as in the preceding run of the reaction. Moreover, in accordance with this invention, the dissolution of the catalyst into the product during the separation of the catalyst from the reaction mixture is almost negligible and since the loss of the catalyst is thus minimized, butadiene or isoprene can be dimerized into alkadiene advantageously on a commercial scale.
The solvent to be employed in the process of this invention must not react readily with the starting material butadiene or isoprene and must be capable of separating the reaction mixture into an alkadiene-containing layer and a solvent layer containing the catalyst. In addition, the solvent must be chemically stable. When solvents such as ethylene glycol, diethylene glycol, nitromethane or the like, are employed in place of sulfolane in the process of this invention, the catalyst can be separated from the reaction mixture. Such solvents, however, are not satisfactory from a commercial viewpoint because the solvents react easily with butadiene or isoprene. When solvents such as acetone, tetrahydrofuran, dimethylformamide, ethyl acetate, t-butanol or the like, are employed in place of sulfolane, the product alkadiene is readily miscible with the solvent to form a homogeneous layer and, therefore the use of phosphine compounds of general formula (I) does not result in catalyst separation from the reaction mixture. If, in the process of this invention, a triarylphosphine such as triphenylphosphine, tri-n-butylphosphine, etc. or a trialkylphosphine is employed in place of a phosphine compound of the above-mentioned general formula (I), the catalyst dissolves into the product, i.e., dimeric alkadiene, so that the above-mentioned advantages in connection with the separation of the catalyst would not be realized.